The application of freeze point depressants on roadways has long been a method of combating the formation of ice. Traditionally, dedicated maintenance vehicles have applied anti-icing solid or liquid chemicals to areas that have a high risk for developing ice. It is important to apply these anti-icing chemicals to the roadway before freezing occurs, as this prevents a bond from forming between ice and the roadway. Freeze-point depressants do this by depressing the freezing point of the liquid on the roadway, much as the anti-freeze in a car radiator prevents it from freezing.
To do this well, a highway agency needs to know whether the current road conditions warrant the application of chemicals. If the road surface has an adequate concentration of chemicals for the current conditions, the application of additional freeze-point depressant is unnecessary, costly, and has an impact on the environment. Road Weather Information Systems (RWIS) and their associated pavement sensors are one cost-effective way for highway agencies to monitor current road conditions, without sending personnel into the field. Many RWIS systems can send information on the current road conditions to a centralized traffic management center, where decisions on the application of additional freeze-point depressant can be made.
There are also some highway sites, such as bridges and overpasses, which typically freeze long before the rest of the roadway. Since the expense of sending a truck with anti-icing chemicals to such a site is high, many highway agencies are installing fixed anti-icing systems. These systems automatically determine the most opportune time to spray, based on the current local conditions as reported by pavement and other RWIS sensors. One of the most important parts of the RWIS system is the pavement sensor, as it allows the determination of the current conditions of the roadway.
In its simplest form, the pavement sensor can consist of a thermometer that measures the temperature of the road surface. Measuring the temperature alone does not give enough information to determine if ice will form, however. This is because the exact concentration of the liquid present on the roadway is not known. For instance, the road temperature may be near 0° C., the freezing point of water. This may mean that the formation of ice is probable; however, previous applications of anti-icing chemicals may have depressed the freezing point of the liquid on the roadway. Precipitation and its runoff may also have diluted the anti-icing chemicals previously applied to the road. To most accurately gauge the current freeze point of the roadway, a sample of the actual liquid on the roadway needs to be analyzed. One method of doing this is to freeze a small sample of solution on the roadway and determine its freezing point. Such a sensor is known as an active sensor, because it actively changes the state of the liquid that is on the road surface.